L1WriteSynchronizer can cause locking with concurrent gets when value is not present

This has been seen previously only on IBM JDK 6 ISPN-3889, however an end user also now can reproduce this issue https://community.jboss.org/thread/236590

The issue is that we don't set the remote lookup value to be null when nothing is found and thus any threads that are waiting for the remote lookup were never notified of this.